Stephen King says that novels (or any work of art) are never finished, only abandoned -- but I think sometimes you DO feel done. People have asked me how you know when you are, and I've answered that question differently than I'm about to now -- even here on this blog when no one was asking! Maybe the answer is different for each book.
The novel I sent to my agent last week (for the fourth time!) feels really and truly done because I've stopped thinking about it. Oh, I still wonder each time I check my email if I'll have heard back from her --but I would be surprised if she asked me to do anything else. And all the space in my head that was taken up imagining scenes or revising them has been wiped clean.
Sometimes after finishing something it feels like a loss, but now I'm glad to be able to concentrate on wherever I am and whatever I'm doing -- like weeding my herb garden:
The mint is from last year, the window frame was given to me by someone here, so I can start seeds under glass. And maybe preparing the ground to sow seeds is where I am in my writing now, too: not ready to start a new novel yet but getting ready -- another way I know I'm done with this one!